Generally, switching converters provides a DC output voltage from an input voltage. Circuit topologies provide a variety of approaches, including pulse width modulation (PWM) and pulse frequency modulation (PFM) topology converters. The choice of topology is determined by the needs of a particular application.
OCP is typically provided in these circuits and systems to protect the components from permanent damage in the event too much current is flowing in the circuitry. The known circuit topology presently being used must be considered in order to explain the OCP approaches. In converter applications where a PWM scheme is used, a constant duty cycle pulse is coupled to the primary switch. Feedback or other control is used to control the width of the pulse so that the output voltage is maintained at a desired level. If the output is rising above the desired level, the width of the pulses to the primary switch may be reduced, thereby reducing the amount of energy transferred to the secondary circuit by the transformer. If the output is falling below desired levels, the width of the pulse may be increased, thereby increasing the amount of energy transferred to the secondary circuit, which can then apply that energy to increase the voltage at the output.
The use of the PWM control on the primary switch also provides rapid overall control of the circuit. If a current monitor detects an over-current condition, that is, current flowing in the circuit exceeding a predetermined threshold, the control circuit can quickly reduce the pulse width of the pulses applied to the primary switch to zero, effectively shutting down the current flowing in the circuit and immediately protecting the components.
However, in another popular circuit topology for switched-mode power converters, this rapid response to an over-current situation to provide OCP is not possible. In this circuit topology, referred to as an “LLC half-bridge” topology, the control is provided using PFM. In PFM converters, an approximate symmetric duty cycle of 50% for the “on” times for the high and low drivers is used; a short “dead time” may also be provided between the high-side driver “on” and low-side driver “on” to prevent any shoot-through currents from flowing. The duty cycle of the pulses is then modulated to control the energy transferred from the primary circuit of the transformer to the secondary circuit. In the inductor-inductor capacitor LLC half-bridge, a high-side driver transistor is coupled in series with a low-side driver transistor and an output is taken at the point between them. A supply voltage is coupled to the high-side driver and a ground reference is coupled to the low-side driver. The output is coupled into the LLC circuit and the voltage output of the converter is taken across the capacitor. In this manner, the high-side and low-side driver transistors are switched alternately on and off by an oscillator, usually a voltage controlled oscillator (VCO). When the high-side driver is on, and the low-side driver is off, energy is supplied into the LLC circuit to create an output voltage. When the high-side driver is off, and the low-side driver is on, energy is discharged into ground. In this manner current can continuously flow. The switching times are arranged preferably so that the two drivers are not active at the same instant, providing a “dead-time” between the devices and a smooth switch over. However in converters using this topology, when an over-current condition is detected, it is not possible to rapidly reduce the energy flow, because the VCO requires at least one and perhaps several cycles to respond to a change in the output frequency desired. Basically, the circuit is a voltage controlled circuit and so an averaging of the output current over some interval must be used in order to regulate the output.
Present methods to provide OCP in the LLC half-bridge topology use a variety of approaches. Typically a controller IC, sometimes called a “resonant converter controller IC”, provides the switching signals to the high and low-side drivers, and often includes the VCO and various control circuits. Sometimes, a predriver integrated circuit may be used with the controller IC and that pre-driver IC then drives the driving transistor gates. Typically one input to the controller IC is a current sense (CS) input. This input pin is used to sense current, or more typically provides a voltage corresponding to the output current, depending on the circuit topology used. In one known approach, a combination of principles is applied to provide OCP for a half-bridge application. The CS input pin coupled to the transformer primary must sense a voltage, since the LLC half-bridge is essentially a voltage controlled circuit. (In contrast, for a PWM application the current sense input pin can sense current to the primary, as the PWM topology is a current controlled circuit.) When the voltage on the input CS pin exceeds a certain threshold voltage, an internal switch is activated and an external capacitor in an external RC circuit is discharged; current into another input pin will then become higher. As the current into this second external pin increases, the VCO frequency is increased; alternatively if the CS input is lower than another threshold voltage, the external capacitor charges, and current into the input pin will become lower, this results in a lower VCO frequency.
Another approach applied to OCP in this prior approach implements a “latch off” function. When the voltage sensed at the input current sensor pin increases to a much higher threshold, for example 1.5V, a second comparator sets a latch and shuts down the circuit. Another circuit implements a “blanking time” function. When an internal comparator senses a voltage on an external delay capacitor greater than a very high threshold, say 3.5V, the controller IC device enters into an “auto-restart” imode. So there are three approaches to an over current situation that require at least three pins, and three external resistors and at least two external capacitors.
In another similar known controller device, when the CS input exceeds a certain threshold, the circuit will change the VCO operation from being responsive to an output feedback voltage to being responsive to a voltage from an RC external compensation circuit, until the condition passes. Again several external resistor and capacitor components are used with a comparator to put the device in a latch off condition when the current sense voltage exceeds a threshold. An auto-restart mode is triggered in this device when an over-current indicated by a voltage at the current sense input causes an RC network to charge to more than a certain voltage, meaning the condition exists for a certain time determined by the charging time of the RC network. This device requires four pins and even more external components in addition to the controller IC to implement the OCP.
A first disadvantage of these approaches is that they require added external components to implement the OCP for the driver controller device. As devices become more and more integrated and the trend to increasing product miniaturization continues, board area for a practical device becomes critical and it is, therefore, undesirable to require external components to implement an IC controlled circuit. Further, the known approaches require a number of pins in the controller IC. As system functions become more integrated, the number of pins available for a particular usage is limited and so this characteristic is also undesirable.
A continuing need thus exists for an improved circuit and methods to provide OCP protection for a converter circuit.